The present invention relates to a method for connecting electronic devices and a connecting cable.
As shown in FIGS. 12(a) and (b), to transmit a signal from a first electronic device la to a second electronic device 1b, a connection is provided as shown in FIG. 12(a) or FIG. 12(b), using a connecting cable 4 in which a signal line 2 is shielded by a first external conductor 3a. 
In the connection of FIG. 12(a), one end of the signal line 2 is connected to the signal output 5 of the first electronic device 1a, the other end of the signal line 2 is connected to the signal input 6 of the second electronic device 1b, one end of the first external conductor 3a is connected to a frame 7, which is the reference potential of the first electronic device 1a, and the other end of the first external conductor 3a is connected to a frame 8, which is the reference potential of the second electronic device 1b. 
In such connection, if there is no potential difference between the frames 7 and 8, good signal transmission can be realized, but, if the signal level is low and there is a potential difference between the frames 7 and 8, then noise mixes into the signal input 6 of the second electronic device 1b. 
In this case, one-point grounding connects the first external conductor 3a only to the frame 7 of the first electronic device 1a, without connecting the other end of the first external conductor 3a to the frame 8 of the second electronic device 1b. 
However, assuming the case in which the first electronic device 1a and the second electronic device 1b are spaced a long distance apart from each other and low-frequency signals (the frequency band is several tens KHz to several tens MHZ) and high frequency signals (the frequency band is several tens MHZ or higher) are transmitted by the signal line 2, or the case in which a digital signal of a high-frequency is transmitted, the one-point grounding as shown in FIG. 12(b) problematically increases the level of undesired radiation of high-frequency signals from the first external conductor 3a into the air.
In this conventional example, the description has been made to one connecting cable 4 by way of example, but, in the connection by a plurality of connecting cables which are placed in parallel between the first electronic device 1a and the second electronic device 1b, there is a problem that noise radiated from the respective juxtaposed connecting cables 4 interferes with each other to further increase the undesired radiation level.
It is the object of the present invention to provide a connection method and a connecting cable, which enable good transmission of low-frequency signals as well as lowering the radiation level of high-frequency signals in such case as described above.
The method for connecting electronic devices of the present invention is characterized by connecting a first electronic device 1a and a second electronic device 1b with a first external conductor being one-point grounded, and connecting an end of a second external conductor on the second electronic device 1b side to reference potential of the second electronic device 1b for high-frequency signals, thereby providing a connection between the electronic devices, and enabling good transmission of low-frequency signals as well as lowering the radiation level of high-frequency signals.
The method for connecting electronic devices as set forth in a first embodiment is characterized in that, to connect electronic devices by a connecting cable, the part between the ends of the signal line of the connecting cable is shielded by a first external conductor, one end of the first external conductor is connected to the reference potential of one electronic device of the electronic devices, the first external conductor is shielded by a second external conductor, the second external conductor is connected to the reference potential of the other electronic device, whereby the reference potential of one electronic device and the reference potential of the other electronic device are coupled through the stray capacity between the first external conductor and the second external conductor.
With this arrangement, the first external conductor is one-point grounded to the first electronic device preventing the difference in the reference potential levels of the first and second electronic devices from being brought into the second electronic device, whereby good transmission of low-frequency signals can be accomplished, and the impedance of the first external conductor to high-frequency signals decreases, reducing undesired radiation.
The method for connecting electronic device as set forth in a second embodiment is characterized in that, to connect electronic devices by a connecting cable, the signal lines of a plurality of connecting cables are respectively shielded by a first external conductor between the ends thereof, one of the respective first external conductors is connected to the reference potential of one electronic device of the electronic devices, the respective first external conductors are shielded by a common second external conductor, and the second external conductor is connected to the reference potential of the other electronic device, whereby the reference potential of one electronic device and the reference potential of the other electronic device are coupled through the stray capacity between the first and second external conductors.
With this arrangement, the respective first external conductors are one-point grounded to the first electronic device preventing the difference in the reference potential level between the first device and second electronic devices from being brought into the second electronic device, whereby good transmission of low-frequency signals can be accomplished, and the impedance of the respective first external conductors decreases, reducing undesired radiation.
The method for connecting electronic devices as set forth in a third embodiment is characterized in that, to connect electronic devices by a connecting cable, the signal lines of a plurality of connecting cables are respectively shielded by a first external conductor in the part between the ends thereof, one end of the respective first external conductor is connected to the reference potential of one electronic device of the electronic devices, the respective first external conductors are electrically connected to each other in the other end thereof, the respective first external conductors are shielded by a common second external conductor, and the second external conductor is connected to the reference potential of the other electronic device, whereby the reference potential of one electronic device and the reference potential of the other electronic device are coupled through the stray capacity between the first external conductor and the second external conductor.
With this arrangement, because the first external conductors are electrically connected to each other, in addition to the construction of the second embodiment, the system of each first external conductor for high-frequency signals is stable as compared with the case in which the first external conductors are not positively made equipotential in the other end thereof, and no independent standing wave occurs in the respective first external conductors.
The method for connecting electronic devices as set forth in a fourth embodiment is characterized in that the length of opposition between the first and second external conductors is adjusted according to a frequency for which undesired radiation is to be suppressed.
The method for connecting electronic devices as set forth in a fifth embodiment is characterized in that the adjustment is performed by connecting, between the first and second external conductors, a element having a capacitance according to the frequency for which undesired radiation is to be suppressed.
With this arrangement, the undesired radiation of the high frequency band can be reduced by the action of a stray capacity between the first and second external conductors, and the cutoff frequency of the low frequency band for high frequencies is adjusted by the capacitor element connected between the first and second external conductors to suppress undesired radiation.
The method for connecting electronic devices as set forth in a sixth embodiment is characterized in that, to connect electronic devices by a connecting cable, signal lines of a plurality of connecting cables are respectively shielded by first external conductors between the ends thereof, one end of the respective first external conductors is connected to the reference potential of one of the electronic devices, the other ends of the respective first external conductors are electrically connected to each other, the respective first external conductors are shielded by a common second external conductor, the second external conductor is connected to reference potential of the other electronic device, and the other ends of the respective first external conductors are electrically connected to each other, and covered with a third external conductor which contacts the outside of a bundle of the first external conductors of a plurality of connecting cables and opposed-to the second external conductor, whereby the reference potential of one electronic device and the reference potential of the other electronic device are coupled through a stray capacity between the second external conductor and the third external conductor.
With this arrangement, the bundle of the first external conductors of a plurality of connecting cables is covered with a third external conductor, thereby connecting the first external conductor to the reference potential of the second electronic device by the stray capacity generated between the second and third external conductors, and thus the stray capacity generated between the second and third external conductors does not depend on the diameter of the respective first external conductors.
The method for connecting electronic devices as set forth in a seventh embodiment is characterized in that the length of opposition between the second and third external conductors is adjusted according to the frequency with which undesired radiation is to be suppressed.
The method for connecting electronic devices as set forth in an eighth embodiment is characterized in that the adjustment is performed by connecting, between the third and second external conductors, a capacitor element having a capacitance corresponding to the frequency with which undesired radiation is to be suppressed. With this arrangement, the undesired radiation of the target frequency can be selectively suppressed.
The method for connecting electronic devices as set forth in a ninth embodiment is characterized in that at least one of the second and third external conductors is a braided wire.
The method for connecting electronic devices as set forth in a tenth embodiment is characterized in that a sheet made up of a first and second conductor sheets opposed to each other through an insulation film is wound around the connecting cable, making the inner first conductor sheet as the third external conductor and the outer second conductor sheet as the second external conductor, whereby the reference potential of one electronic device and the reference potential of the other electronic device are coupled through the stray capacity between the first conductor sheet and the second conductor sheet.
With this arrangement, the number of steps in the terminal process can be reduced and large stray capacity can be obtained by thinning the thickness of the insulation film of the sheet.
The method for connecting electronic devices as set forth in an eleventh embodiment is characterized in that, to connect electronic devices by a connecting cable, signal lines of a plurality of connecting cables are respectively shielded by a first external conductor between one end and the other end thereof, one end of the respective first external conductors is connected to reference potential of one electronic device of the electronic devices, and the other ends of the respective first external conductors are electrically connected to each other and connected to reference potential of the other electronic device, thereby preventing a standing wave from being independently generated in the first external conductor of each connecting cable.
With this arrangement, as compared with the case in which the first external conductors are not positively made equipotential to each other at the other end thereof, the system of each external conductor toward high-frequency signals becomes stable, and a standing wave does not separately occurs in the respective first external conductors, so it is suitable for transmission of digital signals.
The connecting cable as set forth in a twelfth embodiment is characterized by comprising a first external conductor for shielding signal lines between one end and the other end thereof, and a second external conductor opposed to the first external conductor through an insulator and for shielding the first external conductor, wherein the first external conductor on one end of the signal line is connected to reference potential of one electronic device connected by the signal line, and the second external conductor on the other end of the signal line is connected to reference potential of the other electronic device.
The connecting cable as set forth in a thirteenth embodiment is characterized by comprising a first external conductor for shielding the part between the ends of a signal line, and a second external conductor opposed to the first external conductor through an insulator and for shielding part of the other end of the first external conductor, wherein the first external conductor on one end of the signal line is connected to reference potential of one electronic device connected by the signal line, the second external conductor on the other end of the signal line is connected to reference potential of the other electronic device, and at least one parameter of the length of opposition between the first and second external conductors, the electrode distance between the first and second external conductors, and the material of the insulator are set according to a frequency with which undesired radiation is to be suppressed.
The connecting cable as set forth in a fourteenth embodiment is characterized in that there is provided a capacitor element connected between the first and second external conductors, and the capacitance of the capacitor element is set to a capacitance corresponding to the frequency with which undesired radiation is to be suppressed.
The connecting cable as set forth in a fifteenth embodiment is characterized in that the second external conductor is a braided wire, and the distal end of the second external conductor of a braided wire is folded back to the side of one end of the signal line.